In general, the present invention relates to signal receivers employing video amplifiers and more particularly to an aperture correction system for video amplifiers whereby the transient response of the amplifier to sudden changes in an applied signal is enhanced. It is the transient response to sudden changes in applied signals which appears to most significantly indicate the performance of a video amplifier in a signal receiver and particularly a television receiver.
In evaluating the transient response of an amplifier stage several terms related thereto are encountered. For example, rise time is usually defined as the time required for an output pulse to rise from 10 to 90 percent of its final value and is inversely proportioned to the bandwidth of the amplifier. Overshoot is the amount by which a transient rise or fall in signal exceeds its final value and is dependent upon the time-delay characteristics of the amplifier as well as the way in which amplification falls off with increasing frequency. Moreover, a preshoot condition exists when a given level of potential decreases by a given amount prior to a transient increase in potential and increases prior to a transient decrease in potential.
In the utilization of video amplifiers for television receivers it has been a common practice to employ some form of peaking circuitry. Normally, shunt peaking, series peaking, or some combination of both is employed to provide compensation for delay and amplitude decreases due to high frequency fall off of a video amplifier stage. Thus, it is not uncommon to find overshoot characteristics in the output signal of a video amplifier stage.
Although the above-mentioned peaking of the video amplifier stages does enhance the reproduction and sharpness of a visual display on a television receiver, it has been found that the reproduced visual image does leave something to be desired. More specifically, it has been found that picture reproduction is enhanced by an aperture correction system wherein preshoots as well as overshoots are simultaneously controlled.
One known aperture correction system is set forth in an article entitled "Horizontal Aperture Equalization" by A. N. Thiele which appeared in "The Radio and Electronics Engineer" Vol. 40, No. 4, Oct. 1970, pages 193-212. Therein, several relatively complex and expensive aperture correction systems are suggested. Also, a more simple correction system having fixed parameters and suitable for video frequencies is discussed.